Hello I’m new here!
I wouldn’t normally venture into Tech Log because I bimble around in a rented 152. My day job in designing and deploying mobile networks, including a couple of onboard cells used in many wide bodies.
Mobile phones adjust their transmission power dynamically depending on how far away they are from the nearest base station(s). At an airport their power will be very low because there’s several BS’s in the terminal. As they get further away the need to up the power.
Smart phones especially are in continuous communication with the network even when they are not being used. Push email, weather apps, facebook, Tw!tter and the like exchange data on an almost continuous basis.
A Smart phone is trying to keep two radio streams open, one for the voice and sms service and another for data. A mobile onboard an aircraft quickly gets confused. It finds itself moving too fast to be a able to locate and handover to another base station successfully and gets into a panic. As the distance between phone and ground increases so will the transmission power. It’s also struggling with the data connection. In normal operation the phone will use the fastest data service available which is normally HSPA or 4G, as the signal degrades it will fall back onto slower technologies with better reach; UMTS, then EDGE, then GPRS. In the air this signal degradation happens a lot faster due to the vs and forward speed. This also stresses the phone as it hurriedly tries to switch to slower technologies. Bursting data, renegotiating, bursting data again etc.
Once out of range of the base stations it will periodically search for a connection, sometimes at maximum power. Also, don’t forget the communications that might be going on with the PAX’s other technology. Bluetooth headsets, tethering and portable WiFi hotspots could all be exchanging information with each other.
The point on the previous page about airlines being RF immune is not correct. For starters they use RF to communicate! Although everything should be shielded perfectly that changes with age and maintenance interventions. Degraded insulation, temperature changes, stretch and flex will all open an aircraft up to potential interference. I know of a case, unfortunately not documented or scientifically proven in which a mobile was firmly blamed for slowing down the clock speed of a FADEC on a Williams FJ44 by a few MHz. Having a base station onboard will actually significantly reduce the chances of interference as the transmission power needed to communicate a few metres is very small. There’s also no cellular handover on onboard systems meaning phones don’t search around for other base stations.